Limit SDF dowscaling

Given the following style.json render test example:

{
  "version": 8,
  "metadata": {
    "test": { "height": 256, "width": 256 }
  },
  "center": [ 0, 0 ],
  "zoom": 0,
  "sources": {
    "point": {
      "type": "geojson",
      "data": {
        "type": "FeatureCollection",
        "features": [
          {
            "type": "Feature",
            "properties": { "name": "Napoli", "name_en": "Naples" },
            "geometry": {
              "type": "Point",
              "coordinates": [ 0, 0 ]
            }
          }
        ]
      }
    }
  },
  "glyphs": "local://glyphs/{fontstack}/{range}.pbf",
  "layers": [
    {
      "id": "text",
      "type": "symbol",
      "source": "point",
      "layout": {
        "text-field": ["format",
            ["get", "name_en"], { "font-scale": 48 },
            "Italy", { "font-scale": 16 } ,
            "\n", {},
            ["get", "name"], { "font-scale": 16, "text-font": ["literal", [ "NotoCJK" ]] }
        ],
        "text-font": [
          "Open Sans Semibold",
          "Arial Unicode MS Bold"
        ],
        "text-size": 1
      }
    }
  ]
}

In the example above, text-size is set to 1, while font-scale is set to different values. This generates downscaled SDF textures that produces unreadable results.

Rendered result:

/cc @ansis @ChrisLoer

Use the supplied style.json render test as the reproduction, focusing on text-size 1 and the differing font-scale values that generate downscaled SDF textures. Trace how this example produces unreadable glyphs; done means the rendered text remains readable without changing the requested font-scale behavior.